Warren H. Lewis 269 
oped into a retina, fused with the brain, and so give the appearance of a 
retina regenerating from the brain. 
We have noted that the regenerating eyes vary greatly in size, this is 
dependent in part upon the age of the embryo, but to a greater extent, I 
believe, on the amount of optic vesicle tissue left attached to the brain, 
the smaller the amount left the smaller the eye, the greater the amount 
left the larger the regenerating eye. 
In most of the eyes the retina, the pigment layer, and the optic stalk 
are found and usually in normal proportions. How does this happen that 
so many of the regenerating eyes show such normal proportions of these 
tissues? It is scarcely probable that in cutting the eye away there was 
left each time just the right proportion of retinal, pigment, and optic 
stalk-forming cells. The explanation I have to offer is that some, 
enough at least of each of these kinds of cells were left and that owing 
to a self-regulating mechanism regeneration of each of these tissues, went 
on at rates in inverse proportion to the number of the cells of each left 
behind attached to the brain. A like explanation would be given also 
for the final formation of the transplanted eyes. According to this 
idea of inverse proportional regeneration, if the stump of an optic vesicle 
contained a large number of retinal-forming cells and a very small num- 
ber of those destined to form pigment layer there would be a greater 
regenerative activity of the pigment-forming cells until they, by multi- 
plication, were present in about the same proportion as in a normal eye. 
In some of the eyes this disproportion of the cells of various kinds may be 
so great that a normal eye cannot form. The question naturally arises 
as to whether optic stalk-forming cells might not form retinal or pigment- 
layer cells, or retinal cells, pigment cells, and vice versa. 
The fact that after the optic vesicle is cut away the brain may show 
no indication of injury yet no indication of a regeneration of an eye or 
an optic stalk even be found, the fact that we get regenerating eyes of 
all sizes even days after the operations, the fact that among the trans- 
planted eyes the pieces of brain tissue which were transplanted with 
them differentiate into brain tissue and are not incorporated as a part 
of the eye, would seem strong confirmation that the tissue of the optic 
vesicle and adjoining regions of the brain is already predetermined at 
the time of the operation and that brain tissue does not change into eye 
tissue or eye tissue into brain tissue and that the brain cannot regenerate 
an eye. 
